LU-8999 quota: fix quota iteration interface

LU-8999 quota: fix quota iteration interface

Since zfs 0.7.0, object accounting is maintained by DMU, so that quota
iteration interface should retrieve the information over there.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I3d0744dfb52b1a9088b828bc72d648872ec4d00b
Reviewed-on: https://review.whamcloud.com/28345
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>